#include<stdio.h>

int main(void) {
	freopen("e1.txt", "r", stdin);
	freopen("s1.txt", "w", stdout);

	int i, n;
	double t, soma = 0, media = 0;

	scanf("%d", &n);

	for (i = 0; i < n; i++) {
		scanf("%lf", &t);
		t = 1 / t;
		soma += t;
	}
	media = n / soma;

	printf("%.4lf", media);

	fclose(stdin);
	fclose(stdout);

	return 0;
}
